Democratic Republic of Congo will release about 2,000 prisoners this month in a bid to calm political tension at the start of an election year and reduce overcrowding in jails, the justice minister said on Tuesday.

Human rights groups welcomed the announcement but criticized the government for ignoring scores of prisoners they say have been detained for accusing President Joseph Kabila of trying to stay in power after his mandate ends in December.

Kabila became president in 2001, won disputed elections in 2006 and 2011 and has yet to outline his political future.

Justice Minister Alexis Thambwe said about 1,200 low-level offenders who have served at least one-quarter of their sentences and received approval from the prison director and prosecutor-general would be freed in the coming weeks.

About 800 additional prisoners are to be released following pardons granted last week by Kabila to all prisoners over 70 and who have not committed serious crimes and to members of a separatist religious movement.

Congo, Africa's leading copper producer, has not had a peaceful transition of power since independence in 1960.

Millions died mainly of hunger and disease in a 1998-2003 civil war that drew in more than a half-dozen regional powers.

Thambwe said the measures were aimed at reducing tension before a presidential election scheduled for November and a national dialogue to be held in advance.

"We have taken [these] measures to calm the general situation ahead of the dialogue," Thambwe told Reuters.

The dialogue has been delayed as most major opposition parties refuse to participate, dismissing it as a trap to extend Kabila's mandate beyond the end of this year.

Congo's jails are notoriously overcrowded and hold around 20,000 prisoners, according to the United Nations.

Dolly Idefo, executive director of Voice for the Voiceless, a Kinshasa-based human rights group, said the prisoner release did not go far enough.

"There are political prisoners throughout this country and people who have been detained without any [judicial] decision," he said. The government denies it holds any political prisoners.

At least 40 were killed and hundreds more detained last January in protests against changes to the election code that critics said was a ploy to delay the presidential election.